Various methods of manufacturing contact lenses are known, including spin casting, lathing (for example by diamond turning), and cast molding (for example, using injection molded mold halves). Cast molding of contact lenses involves forming a pair of mold halves (i.e., a first mold half and a second mold half), placing a volume of a contact lens formulation on an optical quality surface of one of the two mold halves, and placing the two mold halves in contact with each other to form a contact lens mold assembly that has a contact lens-shaped cavity containing the contact lens formulation. The contact lens mold assembly is then exposed to conditions to cause the contact lens formulation to polymerize or cure in the contact lens mold assembly. Using cast molding processes on automated high speed manufacturing lines, contact lenses can be manufactured in large numbers, for example, tens of thousands of lenses each day. Increases in the rate of manufacture of contact lenses can be expected to bring down the cost of each lens. It is important, however, that increased rates of manufacture do not compromise the quality of the lenses produced.
One common method of curing the contact lens formulation is to heat the mold assembly in a curing oven. It is advantageous to provide a gas purge in the oven, to prevent oxygen in the air from interfering in the polymerization process; for example, a nitrogen atmosphere can be used. For successful polymerization, it is important that the contact lens formulation is heated rapidly. For example, polymerisation of silicone hydrogel contact lenses can be poor if the contact lens formulation is heated too slowly.
The desire to increase the rate of manufacture of contact lenses means that there is a need to increase the number of contact lens mold assemblies in curing ovens at any given time on a contact lens production line, whilst maintaining the correct manufacturing conditions for all of the mold assemblies.